(a) Sketch a line diagram of an emergency switchboard arrangement, labelling the MAIN components.
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(b) Explain how main electrical power is restored to the emergency switchboard after a blackout, stating the precautions necessary
(a) Emergency Switchboard Line Diagram and Main Components
A line diagram of an emergency switchboard arrangement typically consists of the following main components:
- Emergency Power Source: This can be a dedicated generator or an uninterruptible power supply (UPS) depending on the application. The line diagram will show the connection point for the emergency power source.
- Transfer Switch: This is an automatic or manual switch that disconnects the normal power supply (utility grid) and connects the emergency power source to the emergency switchboard in case of a mains failure.
- Emergency Busbar: This is a conducting bar that distributes power from the emergency power source to various emergency circuits within the facility.
- Circuit Breakers: Individual circuit breakers are connected to the emergency busbar, providing overcurrent protection for each outgoing emergency circuit. These breakers protect the cables and equipment connected to each circuit.
- Isolation Contactors (Optional): In some cases, isolation contactors might be included on the normal power supply side to ensure complete separation from the emergency circuits during emergency operation.
- Control Panel: The line diagram might also show the control panel that houses the logic for automatic transfer during a power outage and may include manual controls for operating the transfer switch.
Here’s a simplified explanation of the components and their connections:
- The emergency power source (generator or UPS) connects to the line diagram at a designated point.
- The transfer switch is positioned between the normal power supply (grid) and the emergency power source.
- During normal operation, the transfer switch connects the emergency busbar to the normal power supply.
- In case of a power outage, the transfer switch automatically (or manually) disconnects the normal power supply and connects the emergency busbar to the emergency power source.
- The emergency busbar supplies power to individual emergency circuits through circuit breakers.
(b) Restoring Main Power and Precautions
Restoring Main Electrical Power:
The process of restoring main electrical power (grid) to the emergency switchboard after a blackout involves the following steps:
- Verify Grid Restoration: Ensure that the normal power supply (grid) has been restored and is stable.
- Prepare for Transfer: On the emergency switchboard, isolate critical loads that might be sensitive to switching transients during the transfer process (if applicable).
- Transfer Switch Operation: Following manufacturer’s instructions, initiate the transfer switch operation to switch the emergency busbar back to the normal power supply. This might involve automatic or manual operation depending on the system design.
- Monitor and Verify: Monitor system parameters like voltage and frequency to ensure a smooth transition. Verify that emergency loads are functioning correctly after switching back to the grid.
- Emergency Power Source Shutdown: Once stable operation on the grid is confirmed, shut down the emergency power source following proper procedures (e.g., cool down period for generators).
Precautions:
- Qualified Personnel: Only qualified personnel familiar with the emergency switchboard operation should perform the restoration process.
- Safety First: Always follow electrical safety procedures, including lockout/tagout, when working on the switchboard.
- Load Management: Consider the capacity of the normal power supply and gradually restore non-critical emergency loads to avoid overloading the grid upon transfer.
- Equipment Checks: After restoring main power, it’s recommended to perform a functional test of the emergency power source to ensure its readiness for the next outage.
By following these steps and precautions, the main electrical power can be safely restored to the emergency switchboard after a blackout, minimizing disruption and ensuring the functionality of critical loads during power outages.